Confuse ten with four - that could be a substantial sum (7)
I believe the answer is:
fortune
'a substantial sum' is the definition.
The answer and definition can be both related to property as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'confuse ten with four' is the wordplay.
'confuse' indicates an anagram.
'with' means one lot of letters go next to another.
'ten' after 'four' is 'fourten'.
'fourten' anagrammed gives 'FORTUNE'.
'that could be' is the link.
Can you help me to learn more?
(Other definitions for fortune that I've seen before include "Big bucks - fate" , "'Chance, luck (7)'" , "'. . . . . . . favours the brave'" , "Fate, destiny" , "Huge amount of money" .)